In the embayments of central and western Japan, the harmful dinoflagellate Heterocapsa circularisquama has caused large scale red tides, which brought on mass mortalities to both natural and cultured bivalves. The alga has bacteria inside the cells and occasionally in their food vacuoles. It is suspected that these bacteria give some effects on the algal ecophysiology. The mutual relationships were investigated between the groth of H. circularisquama and intracellular bacteria with laboratory culture experiments. The five algal strains (Y, UA, UB, A, and I) were isolated from different locations of western Japan. Each strain culture was established with elaborate micro-pipette washings, nevertheless, each culture strain contains intracellular and extracellular bacterial populations, designated as Yb, Uab, Ubb, Ab, Ib, respectively. The extracellular bacteria were presumably released from the algal cells. The total number of bacterial cells in the each algal culture increased during the
… More algal exponential growth phase. According to the final cell yields, the five bacterial populations are divided into three groups, Yb and Uab, Ab and Ib, and Ubb group, respectively. The bacterial growth depended on live algal cells. Growth experiments were carried out on the five H. circularisquama strains with bacteria and a bacteria-free strain, established from the dtrain UB, under the conditions of five different light intensities and in five different strengths of medium. There appeared to be no significant advantages for the growth and survival of the algal strains with intracellular bactera, as compared with the bacteria-free strain. Intracellular bacteria, however, might contribute to H. circularisquama populations in some aspects such as life cycle and toxin production.本研究に用いたH.circularisquamaは,英虞湾(A株),伊万里湾(I株),浦ノ内湾(UA,UB株),八代海(Y株)から分離された5つのクローン株である。各株に由来する細胞内細菌を,AB,IB,UAB,UBB,YBとした。これらをSWM-3培養液中で増殖させ,細菌と藻類の挙動を追った。また,数種の培養液中で細菌の培養を試みた。さらに,各クローン株および無菌株を用いて,光条件と栄養塩濃度を変化させた培養実験を実施することにより,細胞内細菌が本藻の増殖に及ぼす影響を検討した。培養中の細菌数,及び本藻一細胞当たりの細胞内細菌数の変動パターンから,これらの細菌株を3つにグループ分け出来た。すなわち,YBとUAB(培養液中に10^6 cells/mlのオーダー,本藻一細胞当たり10細胞以下),ABとIB(培養液中に10^7 cells/mlのオーダー,10〜100細菌/藻細胞),ならびにUBB(培養液中に10^8 cells/mlのオーダー,10細菌以下/藻細胞)である。また,YBとUAB,ABとIB,UBBの順で,細菌の増殖は生きた藻類細胞に依存した。さらに,種々の培地を用いて細胞内細菌の分離培養を試みたが,UBBが微小なコロニーを形成したのみで,他の細菌は分離不能であった。以上からH.circularisquama細胞内細菌は多様であり,しかも両者の相互関係も多様である事が示唆された。増殖可能な光強度の条件下では,いずれの濃度のSWM-3培地で本藻を培養しても,各クローン株の増殖速度は同じであった。本藻のクローン株間で細胞内細菌が増殖に与える影響に関して顕著な差は認められなかった。本藻は混合栄養を営んでいると考えられるが,増殖に対しての餌の細菌の貢献はさほど大きくないと考えられた。 Less